This invention relates to computer systems, and more particularly to a memory management system used in a fault-tolerant computer having multiple CPUs.
Highly reliable digital processing is achieved in various computer architectures employing redundancy. For example, TMR (triple modular redundancy) systems may employ three CPUs executing the same instruction stream, along with three separate main memory units and separate I/O devices which duplicate functions, so if one of each type of element fails, the system continues to operate. Another fault-tolerant type of system is shown in U.S. Pat. No. 4,228,496, issued to Katzman et al, for "Multiprocessor System", assigned to Tandem Computers Incorporated. Various methods have been used for synchronizing the units in redundant systems; for example, in said prior application Ser. No. 118,503, filed Nov. 9, 1987 now abandoned, by R. W. Horst, for "Method and Apparatus for Synchronizing a Plurality of Processors", also assigned to Tandem Computers Incorporated, a method of "loose" synchronizing is disclosed, in contrast to other systems which have employed a lock-step synchronization using a single clock, as shown in U.S. Pat. No. 4,453,215 for "Central Processing Apparatus for Fault-Tolerant Computing", assigned to Stratus Computer, Inc. A technique called "synchronization voting" is disclosed by Davies & Wakerly in "Synchronization and Matching in Redundant Systems", IEEE Transactions on Computers June 1978, pp. 531-539. A method for interrupt synchronization in redundant fault-tolerant systems is disclosed by Yondea et al in Proceeding of 15th Annual Symposium on Fault-Tolerant Computing, June 1985, pp. 246-251, "Implementation of Interrupt Handler for Loosely Synchronized TMR Systems". U.S. Pat. No. 4,644,498 for "Fault-Tolerant Real Time Clock" discloses a triple modular redundant clock configuration for use in a TMR computer system. U.S. Pat. No. 4,733,353 for "Frame Synchronization of Multiply Redundant Computers" discloses a synchronization method using separately-clocked CPUs which are periodically synchronized by executing a synch frame.
As high-performance microprocessor devices have become available, using higher clock speeds and providing greater capabilities, such as the Intel 80386 and Motorola 68030 chips operating at 25-MHz clock rates, and as other elements of computer systems such as memory, disk drives, and the like have correspondingly become less expensive and of greater capability, the performance and cost of high-reliability processors has been required to follow the same trends. In addition, standardization on a few operating systems in the computer industry in general has vastly increased the availability of applications software, so a similar demand is made on the field of high-reliability systems; i.e., a standard operating system must be available.
It is therefore the principal object of this invention to provide an improved high-reliability computer system, particularly of the fault-tolerant type. Another object is to provide an improved redundant, fault-tolerant type of computing system, and one in which high performance and reduced cost are both possible; particularly, it is preferable that the improved system avoid the performance burdens usually associated with highly redundant systems. A further object is to provide a high-reliability computer system in which the performance, measured in reliability as well as speed and software compatibility, is improved but yet at a cost comparable to other alternatives of lower performance. An additional object is to provide a high-reliability computer system which is capable of executing an operating system which uses virtual memory management with demand paging, and having protected (supervisory or "kernel") mode; particularly an operating system also permitting execution of multiple processes; all at a high level of performance.